Electroconductive article



April 10, 1956 A GAISER ELECTROCONDUCTIVE ARTICLE Filed March 5, 1955 4Q v n Q9272 4: 19mm United States Patent F ELECTROCONDUCTIVE ARTICLERomey A. Gaiser, Toledo, Ohio, assignor to Libbey- Owens-Ford GlassCompany, Toledo, Ohio, :1 cerporafion of Ohio Application March 5, 1953,Serial No. 340,548

3 Claims. (Cl. 117--211) The present invention relates broadly totransparent, electrically conducting films, and more particularly to aglass sheet or plate provided with electrically conducting films ofdifferent characteristics on opposite surfaces thereof, and to a methodof producing such a plate.

It has already been known that a transparent, electrically conductingfilm of tin oxide (such as shown in Patent 2,429,420 to H. A. McMaster,dated October 21, 1947) can be produced on glass surfaces by firstheating the glass to substantially its point of softening and thensubjecting one or both surfaces to the action of a tin compound in fluidform.

However, to date films of this character have found their greatestcommercial utility when used on aircraft glazings, for removing ice,frost and fog by heat, and the aircraft manufacturers have insisted thatglass for this purpose be filmed on one side only. In commercialproduction of such electrical conducting glass this has neCeS- i sitatedgoing to rather intricate and expensive means to eliminate film from theoutboard surfaces of lights that are to be used in glazing sightopenings or windows, including Windshields, in aircraft.

Recently, however, it has been realized that static electricity chargesbuilding up on the glass and plastic surfaces of aircraft canopies causeserious problems in connection with radio reception and transmission toand from these aircraft. This is particularly true on the newer highspeed aircraft in which the antenna is placed within the cockpit inorder to reduce air drag. Also, with the higher speed aircraft, thegreater impingement energy of the air particles on the non-conductingsurface of the aircraft, results in larger static charges. In fact,these charges become so great that not only is radio reception andtransmission almost totally impaired, but corona discharges may occuraround metal-housed instruments within the cockpit. It has been foundthat the electrostatic charge on a high-speed aircraft windshield mayproduce corona and interference effects are far as eleven feet distantfrom the windshield surface.

It is an important aim of the present invention to provide a windshieldor other window which has on its outboard surface a transparent,substantially colorless, electrically conducting film of good opticalproperties and a sufiiciently low resistivity for the purposes ofelectrostatic dissipation.

Another object is the provision of a relatively thin outboard film ofthe above character that exhibits stable resistivities.

Another object is to provide a window or windshield which is providedwith a transparent electrically conducting film of uniform thickness andrelatively high resistivities for electrostatic dissipation on onesurface thereof, and a transparent electrically conducting film ofrelatively low resistivity, for defrosting, on the opposite surfacethereof.

Another object is the provision of a method of producing windows orWindshields of the above character in 2 which the relatively lowresistance film is applied to one surface of a glass sheet, while therelatively high resistance film is being simultaneously applied to theopposite surface thereof, under controlled conditions.

Still another object is to provide a method of producing such windows orWindshields in which one side of a heated glass sheet is sprayed for apredetermined length of time with a predetermined volume of air andspray solution to produce a transparent electrically conducting film ofrelatively low resistivity, while at the same time the opposite side ofthe sheet is being sprayed for the same length of time and with the samevolumes of air and spray solution to produce a transparent electricallyconducting film of relatively high resistivity.

Further objects and advantages of the invention will become moreapparent during the course of the following description, when taken inconnection with the accompanying drawings.

in the drawings, wherein like numerals are employed to designate likeparts throughout the same:

Fig. l is a perspective view of a filmed light of electricallyconducting glass produced in accordance with this invention;

Fig. 2 is a fragmentary sectional view taken substantially along theline 2-2 in Fig. 1;

Fig. 3 is a vertical, sectional view through one type of heating furnacewhich may be used. in carrying out the method of the invention;

Fig. 4 is an end view of an apparatus that may be used to spraythe glasssheet after it has been heated; and

Fig. 5 is a side elevation of an aircraft enclosure or canopy glazed inaccordance with this invention.

An electrically conducting coating that has a sufficiently lowresistivity for static dissipation can be produced in a number ofclifierent ways. In fact, I have found that, by removing the maskingdevices or hoods that have heretofore been used in commercial productionto protect the opposite side of the sheet when one side only is beingsprayed for deicing purposes, a back spray of the filming compound willform a film on the reverse side of the sheet that could be consideredadequate from the standpoint of static dissipation (50,000 to 500,000ohms).

However, such a film is inadequate from an optical standpoint because,since it is formed under uncontrolled conditions, it is usually spotty,foggy and almost certainly of improper thickness to give the lightreflection characteristics necessary for aircraft glazings.

It is a premise of this invention that the film on the reverse oroutboard side of the glass (the static dissipating film) must be appliedunder controlled conditions. In other words, according to the invention,at the same time a deicing film is being applied to the inboard surfaceof a windshield light in a manner to achieve a resistivity of perhapsohms per square, another film is being applied to the outboard surfaceof the same light or pane in a manner to provide a resistivity in theorder of 100,000 ohms per square.

By in the order of in this connection is meant a resistivity that mayvary from 10,000 ohms to 1,000,000 ohms, or 1 megohm per square area.These values are given for ideal static dissipation purposes only. It isconceivable that within the range given it may be necessary'to widelyvary the relative resistances of the films on the opposite sides of theglass pane to meet varying conditions of use.

Referring now more specifically to the drawings, there is disclosed inFigs. 1 and 2 a pane of glass 10 filmed in accordance with theinvention, and in Figs. 3 and 4 apparatus which may be employed toproduce the desired films on the glass.

Thus, the glass sheet 10, which may be employed as opposite surfaces '25the outboard pane in an aircraft windshield, is provided with a film llof relatively low resistivity on the side of the pane that is to becomethe inboard surface and a film 12 of relatively high resistivity on theopposite or outboard surface of the pane.

In order to produce such films, the glass sheet after being thoroughlycleaned may be suspended as shown n Fig. 3 from tongs 13 hung from acarnage 1 4 which is provided with wheels 15 running on a monorail 16.The carriage 14 is then moved along the rail 16 tocarry it into afurnace 17, within which it can be heated to the temperature necessaryfor filming.

The furnace 17 will be heated in any suitable manner, such as by meansof electric resistance heaters '18, and may be provided with'balfies 19to provide a more uniform temperature throughout.

Within the furnace, the sheet 10 is heated to substantially thesoftening point of the glass. Actually, of course, glass varies and hasno precise softening point, and for this reason it is not practicable togive any exact temperature requirements for the heating step. However,in securing substantially colorless, transparent,electrically conductingsurfaces on average glass, I have found that exposure to temperaturesbetween 500 degrees to 700 degrees centigrade for from 1 to 10 minutesgives very satisfactory results.

Obviously, these temperature and time factors are infiuencedby the typeof glass, and should run higher as .the softening point of the glassrises. When using sodalirn'e-silica glass in thick automotive plate, Iprefer to heat the plate for two minutes at 1200 degrees Fahrenheit and,after it is withdrawn from the furnace, to position it between the sprayguns Z0 and 21 where the of the sheet will be simultaneously sprayedwith a filming solution to provide electrically conducting coatings ofthe desired resistivities.

Various tin compounds, including the tin halides, dis solved in suitablesolvents can be used as the spray solution to produce the electricallyconducting films and, generally speaking, I prefer stannic tetrachloridein an organic solvent such as isopropyl alcohol for the purpose.

In order to obtain a uniform coating of the spray solution over thesurfaces of the sheet 10, it is desirable to reciprocatethe sprays backand forth over the sheet, and for this purpose the spray guns 20 and 21are mounted .on wheeled carriages .22 and 23 running on tracks 24 and25. The carriages 22 and 23 are moved back and forth over the tracks asufficient number of times to give films of the desired thickness on theglass surfaces as shown at 11 and 12 in Fig. 2. The filmed sheet ,isthen preferably cooled in the atmosphere, at room temperature, or it maybe otherwiseand more rapidly and uniformly chilled, after which hisready for use.

Ideally, in connection with aircraft glazings, it is desired to film theinboard glass surface of the outboard glass pane to a low resistivity inorder that sufficient current may be passed through the film at anavailable voltage to produce heat fordeicing purposes. However, a filmthat has a:resistiv ity in the order of 100 ohms (10 10.1000 ohms persquare) is too good a conductor to allow the passage of radio frequencyenergy. In other words, such a .film would ground out radio frequencyenergy, thus interferwith the transmission of radio signals to or fromthe airplane, through that glass area. For this reasonit may bedesirable to coat some surfaces of an. aircraft .canopy, such as is:shown in Fig. .5 and which may embody a windshield .26 and side andrear windows 27 and .28 respectively, for deicing, while other areas ofthe canopy are .filmed for electrostatic dissipating alone. Thus, at oneand the same time, electrostatic charges on the glass tcanopy :canv beeliminated, .cleicing can he achieved :and radio frequency energy willbe allowed entrance and -.-exit from the canopy.

The areas coated for static dissipation alone, will be coated on theoutboard surfaces. However, according to the preferred embodiment ofthis invention the areas coated for deicing and electrostaticdissipation will be coated on both the inboard and outboard surfaces ofthe outboard pane.

I have employed two types of coatings of films for static dissipationpurposes, one of these is comparatively thick (approximately /2 to wavelength of light at 5600 angstroms), but of high resistance; while theother is much thinner (less than A wave length of light at 5.600angstroms) and'is also of high electrical resistance. The purpose ofchoosing such thicknesses is that thicknesses that are multiples of /2wave lengths on soda-lime-silica glass give lower light refiections'thando thicknesses that are multiples of M1, wave length. However,thicknesses below A Wave length will also give lower light reflection.

The first or thicker type of film is more iridescent than the second orthinner type, but offers greater abrasion resistance to cleaning and toforeign particle impingement. I can produce this thicker film by-use ofa 10% solution of stannic chloride in isopropyl alcohol, to which aniron compound has been added as a conductivity inhibitor. The ironcontent is adjusted according to the teaching in the copendingapplication of Romey A. Gaiser and James W. McAuley, Serial No. 327,722,filed December 24, 1952, to give a filming solution that will produce afilm of the desired resistivity.

Theseeond method of obtaining high resistivities involves the use ofspray solutions containing smaller amounts of tin. For example, I havesuccessfully used ..5.% to 2% of tin chloride in isopropyl alcoholsolution for this purpose; and spray times with this solution vary from.1 to 20 seconds, depending upon thethickness of the film and thetemperature of the glass.

Originally I found one difficulty with the-thin coating. This involvedthe stability of resistivity. I found that thin, high resistivity films,particularly of one megohm or higher, were apt to have unstableresistivities varying both upward and downward from day to day as much"as several hundred per cent of the original resistivity. The hin filmsappear to have much greater photo conductivc effects than the thickerfilms. However, I also found that the addition of gallium compounds tothe spray solution act .as resistivity stabilizers, and that thin 'filmsproduced from spray solutions containing gallium will maintain theiroriginal resistivity within 25%.

To insure reatest mechanical strength in the finished product, it .isadvisable to spray both surfaces of the hot glass plate withapproximately equal volumes of air and spray solution, when the sprayingof both surfaces is done simultaneously. In practice, therefore, it isreeommended that the deicing surface'fbe sprayed "with atinchloride'isopropyl alcohol solution with a tin halide content fiber-ween10 and 30%,while the opposite surface of the glass is beingsimultaneously sprayed "for the same lengthcf time, either with the samesolution to which the optimum of iron has been added, or with a weaker)-lution containing from .5% to 10% tin compound (with .or without iron)and approximately .'1.0% gallium'metal. The former procedure will givefilmstof .equal thickness .onrboth sides of the glass while the latterwill :giuea relatively thick d i ing film and a relatively thin'statiedisipating film.

lBy spraying both surfaces for the same length of time and byusiugepnwximately thesame amount or :alcohol in the spray solution, thecompression layer on .bothrflurfaces-of the glass will be equal andtherefore minimum ,walfpage and maximum strength will result. Also withthis .method the glass will remain clear, with a minimum of'haze, .andwill have the des ired.,resistivities.

.As indicated aboveit is preferred that the .dei ingfilms haves,resistivigrin the order of 1000111115 {10 2 L000 ohms) While the staticdissipating films haven resistivity of the order \of .1 00,000 ohms(10.000to 1,0,00,-.0.0:0 ohms per square).

Suitable marginally arranged electrodes 29 and 30 can be secured to theopposite surfaces of the glass sheet 10, and in electrical contact withthe films 11 and 12, in any well known manner. The pair of electrodes 29on op posite margins of the glass serve to supply electrical current tothe deicing film 11, While the single electrode 30 provides a groundconnection from the static dissipating film 12.

Although this invention has been specifically described in connectionwith sheets or panes of glass, it will be apparent that it is alsoapplicable to a window pane of plastic to the surface or surfaces ofwhich transparent electrically conducting films can be applied. In fact,it is to be understood that the form of the invention herewith shown anddescribed is to be taken as an illustrative embodiment only of the same,and that various changes in the shape, size and arrangement of parts, aswell as various procedural changes may be resorted to without departingfrom the spirit of the invention or the scope of the subjoined claims.

I claim:

1. In a glazing unit for aircraft or the like, the combination with atransparent pane forming part of said unit of a transparent electricallyconducting film having a resistivity of the order of 100 ohms per squarearea on one surface of said pane, and a transparent electricallyconducting film having a resistivity of the order of 100,000 ohms persquare area on the opposite surface thereof.

2. In a glazing for an aircraft enclosure or canopy including at leastone pane of glass, a. transparentelectrically conducting film having aresistivity of the order of 100 ohms per square area on an inboardsurface of said glazing, and an electrically conducting coating having aresistivity of the order of 100,000 ohms per square area on an outboardsurface thereof.

3. A sheet or plate having opposed vitreous surfaces, a transparentelectrically conducting film of tin oxide of predetermined thickness andhaving a resistivity of the order of 100 ohms per square area on one ofsaid surfaces, and a transparent electrically conducting film of tinoxide of substantially the same thickness as said first mentioned filmbut having a resistivity at least 10 times as great on said oppositesurface.

References Cited in the file of this patent UNITED STATES PATENTS2,118,795 Littleton May 24, 1938 2,429,420 McMaster Oct. 21, 19472,564,706 Mochel Aug. 21, 1951

1. IN A GLAZING UNIT FOR AIRCRAFT OR THE LIKE, THE COMBINATION WITH ATRANSPARENT PANE FORMING PART OF SAID UNIT OF A TRANSPARENT ELECTRICALLYCONDUCTING FILM HAVING A RESISTIVITY OF THE ORDER OF 100 OHMS PER SQUAREAREA ON ONE SURFACE OF SAID PANE, AND A TRANSPARENT ELECTRICALLYCONDUCTING FILM HAVING A RESISTIVITY OF THE ORDER OF 100,000 OHMS PERSQUARE AREA ON THE OPPOSITE SURFACE THEREOF.